obsolete: detect divergent changesets Divergent changeset are final successors (non obsolete) of a changeset who compete with another set of final successors for this same changeset. For example if you have two obsolescence markers A -> B and A -> C, B and C are both "divergent" because they compete to be the one true successors of A. Public revision can't be divergent. This function is used and tested in the next changeset.